Mirror Mounts.

by Tom RyanPrinted in Reflections: July, 2009.

The first telescopic mirror mount I ever saw was part of my 1964 Edmund
“Sky Conqueror” 4” F/10 reflector. I remember
nothing about it, other than I could use it to adjust the mirror’s
tilt. (I was too busy conquering the sky to notice
much else!) The next mirror mount was part of a very much up-market 8”
f/4.5 Cave Astrola. It, too, was adjustable
with springs and wing nuts. I learned a lot about optical system alignment
from that mount and the diagonal mirror’s
mount. Mostly, I learned that there were a lot of things that could become
misaligned in a Newtonian telescope.

Every time I threw the Cave into the car to take it to some observing site,
the optics shifted and needed to be aligned before
I used the telescope. At the time, I just figured that that’s the
way things were. Reflectors need to be constantly
aligned, and Refractors never needed alignment. There were so many big
things to learn in Astronomy, I never wondered
why this should be the case.

I bought the three volume set of Amateur Telescope Making, and learned
that mirror mounts also support the mirror’s
weight. Well, OK. Big mirrors seem to have lots of triangular pads, and
small mirrors have three. My 8” Cave had
three cork pads at the 70% zone. It had three cork-covered retaining clips
to keep the mirror from falling out of the cell
when the telescope pointed down. I carefully adjusted these so they would
not quite touch the mirror’s front surface.
This meant the mirror was basically free to slide around, but I thought
that was normal, too.

Above is a picture of the mount and the mirror, along with a whiffle-tree
mount. Note the three white nylon set screws
in the Cave mount, to the left. These were intended to keep the mirror
from sliding around, but since the mirror blank
had strongly tapered sides, tightening these had the effect of popping
the mirror out of the mount. Even as a teenager, I
knew this was not good. Nevertheless, this was state-of-the-art for amateur
telescopes, and this state of the art didn’t
change until the advent of big, thin Dobsonian mirrors. At which point,
the art got significantly worse.

Around 1981, Doug Nelle got a gigantic 17.5” mirror made by Coulter
Optics, and if I’m not mistaken, it was originally
mounted on a disk made of two layers of 3/4” chip-board, and was fastened
to it using several wraps of duct tape around
its edge. Kinematics tells us that two rigid bodies will contact each other
at three points. I’m still wondering where
those three points were on the Coulter mirrors. Not where they were supposed
to be, I’ll bet.

If you are trying to support a long beam
(like a strip of telescope mirror, or a
twenty-foot long piece of wall molding)
and intend to keep it very straight, you’ll have to support it at
enough points to keep the sag that the beam experiences
between supports below your bend requirements. It helps if your beam is
fairly thick (hence the 6:1 diameter-to thickness
ratio recommended by old telescope makers), and it also helps to have lots
of supports. Clearly, the beam
thickness and number of supports are related.

I was asked to redesign the mount of a telescope mirror which was boosted
above the atmosphere. The original designer
had wanted to keep the number of supports to a minimum (three, that is),
and wanted to keep the mirror adjustable. His
solution consisted of using a light-weighted mirror (he adjusted the beam
thickness) called a double-arch, and three
really big bolts.

The existing design had three bolts under the support arch of the mirror
blank (in yellow in the above drawings) and
three more bolts pressing against the face of the mirror to keep the mirror
in place. They were in-line with the lower
bolts to minimize any bending stresses. The mirror was located axially
by a strip of Teflon between the mirror’s central
hole and the baffle support tube hub. However, the mirror was not prevented
from rotating, so when the rocket was
launched and began to spin up like a bullet for stability, the mirror’s
inertia caused it to spin between the bolts, resulting
in large arcs of mirror coating being removed from the mirror’s front
surface.

The mount had other
problems, believe it or
not. Teflon cold flows
(that is, it acts like
cold molasses) and has
a bump in its expansion
curve near room
temperature. It is,
therefore, not a reliable
locator. The bolt support
beams were
made of Invar for temperature
stability, but
Invar is a very weak
metal, and the beams
kept getting bent upon landing. The bolts and nuts were both made of stainless
steel, which naturally galled and tended
to weld together. Grease is not an option in UV spacecraft.

My solution to this was to epoxy three Invar hemispheres to the back of
the mirror, and to remake the mirror support cell
so that it had three radial V-grooves, into which the hemispheres dropped.
This is a kinematic mount, and the mirror can
be removed and replaced within microns of its old position. The Invar pads
were threaded, and bolts secured the mirror
to the back plate. The system has no adjustment, so I lapped the hemispheres
until the axis of the mirror and telescope
were within 0.003” of each other, measured at the secondary mirror.
Once set, this system never goes out of alignment.
(Unless something catastrophically breaks.)

I really like light-weighted, double arch mirrors. They are lightweight
and stiff, though expensive to make. But what do
you do if your mirror is a thin disk?

At some point in time, from my reading, I made an amazing (to me) discovery.
Mirror mounts not only support the mirror
and orient it, but they also should not impose local bending on the mirror.
Let me explain what I mean by this.

If you are supporting a horizontal beam and several of your support points
are too low, you can simply adjust their height
until the beam is straight. The makers of the Hubble mirror took this approach
when polishing it, face up. They supported
the mirror with dozens of individually adjustable pads, each one tunable
to support the mirror’s local weight.
This is much harder to do on a variable orientation telescope mirror, where
beam sags on the order of millionths of an
inch will affect your image, and local weight on the supports goes from
100% to zero as the mirror’s axis is tilted from
vertical to horizontal.

Whiffle tree mounts are one solution to this problem. They consist of three-point
supports on a rigid plate with a pivoting
support point in the middle. These support points can be supported in turn
in groups of two or three, and this process
can be repeated ad-infinitum. Hence, mirror mounts can be found with 3-,
9-, and 18-support points. They were first
popularized by Hindle in ATM vol. I, and one appears in the first photograph
in this article.

Multiple supports are where things can get tricky. Theoretically, the pivoting
support point pivots with no effort. But in
the real world, pivots have friction and tend to stick in place. In sensitive
weighing balances, pivots are often made using
high hardness, low friction jewels, such as sapphire, to allow the balance
to adjust freely to tiny forces. However,
mirror mounts usually don’t have anything like that level of engineering.
(Scales can also be lifted off their pivots, and
are usually not tilted sideways.) As a result, the mirror cell’s pivot
point can stick at one particular angle, and then the
three support points don’t support equal amounts of the mirror’s
weight. They cause the mirror to bend, locally. I, personally,
saw a “professional” 18-point mirror mount in which only about
half the mirror supports were actually touching
the mirror. Clearly, those pivots had much too much friction. The challenge
is to make the pivot point strong enough to
support the mirror’s weight, and weak enough to bend before the mirror
deflects locally by one-millionth of an inch.

Many Dobsonian
(big,
thin) mirrors use
globs of silicone
sealant to both
retain the mirror
in place and support
it. The silicone
sticks to
the glass very,
very well, and in
bulk, acts like a
soft spring. That
is, it deflects a
lot under little
pressure. This
means that if the
mount that supports
the mirror
is bending because
of heat or
stress, the mount
deformations are
averaged out
over many globs
before they get to the mirror.

The disadvantage of this is that those same soft springs allow the mirror
to tilt by different amounts at different orientations.
The soft springs act in the extreme like a bunch of Slinkies. For amateur
Newtonians, which have fairly large
fields of fairly good definition, mirror tilt is not a major problem, especially
if they are used visually. But for some telescopes,
mirror tilt with orientation can be a very big problem.

The solution most professionals use for mid-size mirrors is to mount the
mirror using beams which are weak in bending
but strong in compression. They also try to place the mounting points at
the mirror’s neutral axis. The neutral axis is the
imaginary surface inside the mirror blank itself which, if the mirror is
bent in any way, bends, but neither compresses nor
stretches.

An example of this kind of mount is shown in the following photograph of
the system used to mount the Zerodur primary
mirror in the Cassegrain telescope used in the Deep Impact mission to Comet
Tempel 1. You can see that the mirror
supports connect to the mirror at the mirror’s neutral axis, the metal
mounts are glued to flats ground on the side of
the mirror, and the supports are angled to prevent rotation or tilt. They
are also cut away so that they act as if they had
pivot points at either end, while still retaining the axial stiffness of
a rigid beam. These particular beams have been wire
EDM’d to shape, but they could as well have been milled or ground
to shape.

At some point, the opto-mechanical technology used in space flight will
appear in amateur telescopes, the state of the art
will advance, and we’ll all get better stuff, including a direct return
on our tax dollars. Life can be very good.